Flow cytometry has many applications in biology. Acquisition of a FACStarPLUS Cell Sorter for the Flow Cytometry Laboratory at The Salk Institute is proposed to support projects studying: 1) the mechanism of gene amplification in tumor cells, 2) the analysis of hematopoietic cell lineages in transgenic mice, 3) the study of thymocyte progenitors, 4) the isolation and study of hematopoietic stem and progenitor cells, 5) the study of lymphocyte surface antigen function, 6) the regulation of lymphocyte surface antigen expression, 7) the mechanisms by which oncogenes regulate cell growth, and 8) the mapping of human chromosomes. The Salk Institute Flow Cytometry Laboratory comprises a cell sorter and a flow microfluorimeter. The existing cell sorter uses basic technology that was developed in the mid-1970's and no longer meets the need for a machine that can sort cells stained with multiple fluorescent probes with high speed and good purity, especially in situations where the input cells express a wide range of fluorescent intensities, the input sample size is small, non-rectangular sorting gates are required, or isolation of the sorted population as single cell cones or in microcultures is desired. The existing sorter is unique in design and cannot be easily upgraded to meet these needs. It can, however, be optimized for sorting chromosomes, an application that requires considerable sorter time and thus impacts the ability of the Flow Cytometry Laboratory to meet the needs of users whose sorting applications involve problems in cell biology. It is proposed to dedicate the existing sorter to chromosome sorting and to acquire a new sorter that will meet the needs of those who wish to apply cell sorting and multiparameter fluorescent analysis to problems of cell biology.